1. Field of the Invention
The present invention relates to a polycarbonate having heterounits and a method for producing the same. More particularly, the present invention is concerned with a polycarbonate comprising a plurality of aromatic polycarbonate main chains, wherein the aromatic polycarbonate main chains collectively contain specific heterounits in a specific amount in the polycarbonate main chains, and a method for producing the same. The polycarbonate of the present invention is advantageous in that not only does it have high transparency and colorlessness as well as high mechanical strength, but also it can exhibit high non-Newtonian flow properties, so that it exhibits high molding melt fluidity. Therefore, the polycarbonate of the present invention is extremely advantageous from a commercial point of view.
2. Prior Art
Polycarbonates have been widely used in various fields as engineering plastics having excellent heat resistance, impact resistance and transparency. Production of polycarbonates has conventionally been conducted by using the phosgene process. However, polycarbonates produced by using the phosgene process have problems in that the production thereof needs the use of phosgene, which is poisonous, and that they contain residual methylene chloride (solvent), which not only adversely affects the thermal stability of the polycarbonates, but also causes corrosion of a mold used for the molding of the polycarbonates. Therefore, recently, polycarbonates produced by using the transesterification process have been drawing attention.
With respect to transesterification polycarbonates, it is known: that almost colorless, transparent transesterification can be obtained on a laboratory scale; however, when the production of transesterification polycarbonates is conducted on a commercial scale, only those having slightly yellowish color can be obtained see "Purasuchikku Zairyo Koza (5), Porikaboneto Jushi (Lecture on Plastic Materials (5), Polycarbonate Resins)", page 66, published in 1981 by The Nikkan Kogyo Shimbun Ltd., Japan!, and that transesterification polycarbonates have disadvantages in that they have many branched structures, so that they have poor strength (danger of brittle fracture is high), as compared to phosgene process polycarbonates see "Kobunshi (Polymer)", vol. 27, p. 521, July 1978)!.
In order to alleviate these problems of the transesterification polycarbonates, various studies have been made on the structure and production process of the transesterification polycarbonates. With respect to the branched structures of the transesterification polycarbonates, it is known that such branched structures are formed as follows. During the progress of the polymerization reaction in the presence of an alkali in the reaction system, the polycarbonate chain being formed suffers a side reaction represented by the reaction formula described below, which is similar to the Kolbe-Schmitt reaction: ##STR1## As is apparent from the above-shown structure formed in the main chain by the side reaction, a branched chain grows and extends through ester bonds. In some cases, such a branched chain forms a crosslinked structure in the final polycarbonate see "Purasuchikku Zairyo Koza (5), Porikaboneto Jushi (Lecture on Plastic Materials (5), Polycarbonate Resins)", page 64, published in 1981 by The Nikkan Kogyo Shimbun Ltd., Japan; and "Porikaboneto Jushi Hando Bukku (Polycarbonate Resin Hand Book)", page 49, published in 1992 by The Nikkan Kogyo Shimbun Ltd., Japan!.
With respect to the structure of the transesterification polycarbonate, it has been attempted to reduce the amount of branched structure in the polycarbonate. For example, Unexamined Japanese Patent Application Laid-Open Specification No. 5-105751 and Unexamined Japanese Patent Application Laid-Open Specification No. 5-202180 (corresponding to U.S. Pat. No. 5,468,836) disclose a technique to obtain a transesterification polycarbonate having no or an extremely small amount of branched structure. Specifically, in these prior art documents, the transesterification reaction is conducted using a specific combination of catalysts, to thereby obtain a colorless, transparent polycarbonate having no or an extremely small amount of branched structure which is formed by the side reaction during the polymerization. Unexamined Japanese Patent Application Laid-Open Specification No. 7-18069 (corresponding to U.S. Pat. No. 5,418,316) proposes a method for producing a polycarbonate, in which, by the use of a specific catalyst, the amount of the above-mentioned branched structure formed by the side reaction similar to the Kolbe-Schmitt reaction is suppressed to a level as low as 300 ppm or less. The polycarbonates disclosed in these prior art documents have high transparency and colorlessness; however, these polycarbonates have problems in that they exhibit poor non-Newtonian flow properties, so that they disadvantageously exhibit low molding melt fluidity.
For solving the above problems, for example, Unexamined Japanese Patent Application Laid-Open Specification Nos. 5-271400 and 5-295101 (each corresponding to U.S. Pat. No. 5,468,836) disclose a transesterification technique in which the formation of the above-mentioned disadvantageous branched structure resulting from the side reaction of the above reaction formula is reduced by the use of a specific catalyst to thereby achieve an improvement in transparency and colorlessness of the formed polycarbonate, whereas the non-Newtonian flow properties of the polycarbonate are improved by intentionally introducing another specific branched structure to the polycarbonate by the use of a multifunctional compound, to thereby improve the properties of the polycarbonate so that it can be advantageously used for blow molding. Further, in U.S. Pat. No. 4,562,242, it is attempted to improve the molding melt fluidity of the polycarbonate by the use of a 5-(dimethyl-p-hydroxybenzyl)salicylic acid as a branching agent. However, the use of the multifunctional compounds as mentioned above has problems in that these compounds promote a crosslinking reaction during the polymerization, so that the final polycarbonate is likely to contain gel.
Therefore, it has been desired to develop a transesterification technique, in which the occurrence of branching of the polycarbonate structure can be controlled without using a multifunctional compound which is likely to cause gelation, so as to produce a polycarbonate which not only has high transparency and colorlessness as well as high mechanical strength, but also exhibits high non-Newtonian flow properties, so that the polycarbonate can exhibit high molding melt fluidity, as compared to the phosgene process polycarbonates.
Further, with respect to the process for producing a transesterification polycarbonate, various improvements have been proposed. For example, with respect to a process in which use is made of a plurality of polymerizers which are connected in series, it has been proposed to use a special type of polymerizer as a final stage polymerizer, such as a special type of horizontal agitation type polymerizer (see Unexamined Japanese Patent Application Laid-Open Specification No. 2-153923) or a twin screw vented extruder (see Examined Japanese Patent Application Publication No. 52-36159 and Unexamined Japanese Patent Application Laid-Open Specification No. 63-23926. However, the techniques of the above-mentioned prior art documents are only intended to promote the removal of phenol from the polymerization reaction system. Therefore, by these techniques, a polycarbonate having a high molecular weight can be easily obtained; however, the obtained polycarbonate is not satisfactory with respect to the properties thereof, such as mechanical properties and molding melt fluidity.
The task of the present invention is to provide a polycarbonate which is advantageous in that not only does it have high transparency and colorlessness as well as high mechanical strength, but also it exhibits high non-Newtonian flow properties, so that it can exhibit high molding melt fluidity.